ASTM D4623-16

Standard Test Method for Determination of In Situ Stress in Rock Mass by Overcoring Method—Three Component Borehole Deformation Gauge

SIGNIFICANCE AND USE
5.1 Either the in situ stresses or the stresses as influenced by an excavation may be determined. This test method is written assuming testing will be done from an underground opening; however, the same principles may be applied to testing in a rock outcrop at the surface.  
5.2 This test method is generally performed at depths within 50 ft (15 m) of the working face because of drilling difficulties at greater depths. Some deeper testing with this gauge has been done, but should be considered developmental. This test method has a long and proven record and considered very accurate relative to many other techniques, both new and old, out there. Other overcoring methods that use instruments that are different, but follow much of the same basic concepts are now available and can go deeper; however, the pros and cons of each method need to be carefully compared to this test method.  
5.3 It is also useful for obtaining stress characteristics of existing concrete and rock structures, such as mass concrete dams, for safety (such as alkali aggregate issues), vetting of computer models, and modification investigations.  
5.4 This test method is difficult in rock with fracture spacings of less than 5 in. (130 mm). A large number of tests may be required in order to obtain data.  
5.5 The rock tested is assumed to be homogeneous and linearly elastic. The moduli of deformation and Poisson's ratio of the rock overcore are required for data reduction. The preferred method for determining modulus of deformation values involves biaxially testing the recovered overcores, as described in Section 8. If this is not possible, values may be determined from uniaxial testing of smaller cores in accordance with Test Method D7012. However, this generally decreases the accuracy of the stress determination in all but the most homogeneous and isotropic rock. Modulus of deformation results may be used from other in situ tests, such as Test Methods D4394 and Test Method D4395, D4971 or othe...
SCOPE
1.1 This test method covers the determination of the ambient local stresses (principal and secondary) in a rock mass and the equipment required to perform in situ stress tests using a three-component borehole deformation gauge (BDG) that was developed by the U.S. Bureau of Mines (USBM); see Note 1.  
1.2 The test procedure and method of data reduction are described, including the theoretical basis and assumptions involved in the calculations.  
1.3 A section is included on troubleshooting equipment malfunctions.
Note 1: The gauge used in this test method is commonly referred to by users as a USBM gauge (U.S. Bureau of Mines three-component borehole deformation gauge).2  
1.4 The values stated in inch-pound units are to be regarded as standard, except as noted below. The values given in parentheses are mathematical conversions to SI units, which are provided for information only and are not considered standard. Reporting of test results in units other than SI shall not be regarded as nonconformance with this test method.  
1.5 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

General Information

• Status: Published
• Publication Date: 30-Nov-2016
• ICS: 93.020 - Earthworks. Excavations. Foundation construction. Underground works
• Technical Committee: D18 - Soil and Rock
• Drafting Committee: D18.12 - Rock Mechanics

Relations

• Replaces: ASTM D4623-08 - Standard Test Method for Determination of In Situ Stress in Rock Mass by Overcoring Method<span class='unicode'>&#x2014;</span>USBM Borehole Deformation Gauge
• Effective Date: 01-Dec-2016
• Refers: ASTM D3740-23 - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-Nov-2023
• Refers: ASTM D3740-19 - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-Oct-2019
• Refers: ASTM D4394-17 - Standard Test Method for Determining In Situ Modulus of Deformation of Rock Mass Using Rigid Plate Loading Method
• Effective Date: 01-Jul-2017
• Refers: ASTM D4395-17 - Standard Test Method for Determining In Situ Modulus of Deformation of Rock Mass Using Flexible Plate Loading Method
• Effective Date: 01-Jul-2017
• Refers: ASTM D4971-16 - Standard Test Method for Determining In Situ Modulus of Deformation of Rock Using Diametrically Loaded 76-mm (3-in.) Borehole Jack
• Effective Date: 01-Dec-2016
• Refers: ASTM D653-14 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Aug-2014
• Refers: ASTM D7012-14 - Standard Test Methods for Compressive Strength and Elastic Moduli of Intact Rock Core Specimens under Varying States of Stress and Temperatures
• Effective Date: 01-May-2014
• Refers: ASTM D7012-13 - Standard Test Methods for Compressive Strength and Elastic Moduli of Intact Rock Core Specimens under Varying States of Stress and Temperatures
• Effective Date: 15-Nov-2013
• Refers: ASTM D3740-12a - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-May-2012
• Refers: ASTM D3740-12 - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-Mar-2012
• Refers: ASTM D653-11 - Standard Terminology Relating to Soil, Rock, and Contained Fluids
• Effective Date: 01-Sep-2011
• Refers: ASTM D3740-11 - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-Sep-2011
• Refers: ASTM D3740-10 - Standard Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in Engineering Design and Construction
• Effective Date: 01-Mar-2010
• Refers: ASTM D7012-10 - Standard Test Method for Compressive Strength and Elastic Moduli of Intact Rock Core Specimens under Varying States of Stress and Temperatures
• Effective Date: 15-Jan-2010

Overview

ASTM D4623-16 - Standard Test Method for Determination of In Situ Stress in Rock Mass by Overcoring Method-Three Component Borehole Deformation Gauge provides a reliable and proven approach for measuring in situ stress within rock masses. Utilizing a three-component borehole deformation gauge (BDG), this test method is widely recognized for its accuracy in both underground and surface applications. The method supports critical geotechnical and structural assessments, contributing to the safety and design of large-scale rock and concrete structures such as tunnels and dams.

Key Topics

• In Situ Stress Measurement: This standard addresses the measurement of ambient local stresses (principal and secondary) within a rock mass, critical for understanding rock behavior before, during, and after excavation.
• Overcoring Technique: Overcoring involves measuring deformation as a small-diameter borehole is relieved from surrounding stress by cutting a larger, concentric core. The BDG records dimensional changes in the borehole, allowing stress calculations.
• Three-Component Borehole Deformation Gauge: Developed by the U.S. Bureau of Mines (USBM), the BDG measures diametral changes at three axes spaced 60° apart, enabling accurate capture of stress relief.
• Data Reduction and Modulus Determination: For reliable stress results, modulus of deformation and Poisson’s ratio are required, typically obtained via biaxial testing of recovered overcores or from validated uniaxial compressive tests.
• Accuracy and Limitations: Most effective within 15 meters of the excavation face, this method is less suited to highly fractured rocks. Homogeneity and linear elasticity of the rock are assumed for best results.

Applications

ASTM D4623-16 is widely used in:

• Tunneling and Mining Engineering: Assists in the design and monitoring of underground excavations by providing critical stress data for stability and support systems.
• Civil Engineering Projects: Applicable to dam safety evaluations and large concrete or rock structures, supporting assessments of structural integrity and investigation of phenomena such as alkali-aggregate reactivity.
• Research and Model Validation: Provides real-world stress data to vet and calibrate computer models of geomechanical behavior or to assess existing conditions for modification studies.
• Surface and Underground Testing: Although developed for underground use, it is also applicable for surface rock outcrops with similar principles.
• Quality Control: Enables ongoing monitoring of stress conditions, ensuring engineering controls remain effective over the life of a project.

Related Standards

• ASTM D653: Terminology Relating to Soil, Rock, and Contained Fluids.
• ASTM D7012: Test Methods for Compressive Strength and Elastic Moduli of Intact Rock Core Specimens under Varying States of Stress and Temperatures.
• ASTM D4394 and D4395: Methods for determining in situ modulus of deformation in rock masses using plate loading.
• ASTM D4971: Test Method for Determining In Situ Modulus of Deformation of Rock.
• ASTM D3740: Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock.

Practical Value

ASTM D4623-16 ensures rigorous, repeatable procedures for stress determination, enhancing geotechnical diagnoses and structural design decisions. Its practical approach to overcoring and deformation measurement ensures high confidence in stress data, crucial for project safety and longevity. The method’s adaptability to surface and underground sites, coupled with clear guidelines for equipment calibration and troubleshooting, makes it a valuable tool for geotechnical engineers, site investigators, and quality assurance personnel.

Keywords

In situ stress, rock mass, overcoring, borehole deformation gauge, ASTM D4623, underground excavation, geotechnical engineering, stress measurement, modulus of deformation, U.S. Bureau of Mines, civil engineering standards, tunnel stability, dam safety, BDG, rock mechanics.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D4623 − 16
Standard Test Method for
Determination of In Situ Stress in Rock Mass by Overcoring
Method—Three Component Borehole Deformation Gauge
This standard is issued under the fixed designation D4623; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope* D3740Practice for Minimum Requirements for Agencies
Engaged in Testing and/or Inspection of Soil and Rock as
1.1 This test method covers the determination of the ambi-
Used in Engineering Design and Construction
ent local stresses (principal and secondary) in a rock mass and
D4394Test Method for Determining In Situ Modulus of
the equipment required to perform in situ stress tests using a
Deformation of Rock Mass Using Rigid Plate Loading
three-component borehole deformation gauge (BDG) that was
Method
developed by the U.S. Bureau of Mines (USBM); see Note 1.
D4395Test Method for Determining In Situ Modulus of
1.2 The test procedure and method of data reduction are
Deformation of Rock Mass Using Flexible Plate Loading
described, including the theoretical basis and assumptions
Method
involved in the calculations.
D4971Test Method for Determining In Situ Modulus of
1.3 A section is included on troubleshooting equipment DeformationofRockUsingDiametricallyLoaded76-mm
(3-in.) Borehole Jack
malfunctions.
D6026Practice for Using Significant Digits in Geotechnical
NOTE1—Thegaugeusedinthistestmethodiscommonlyreferredtoby
Data
usersasaUSBMgauge(U.S.BureauofMinesthree-componentborehole
D7012Test Methods for Compressive Strength and Elastic
deformation gauge).
Moduli of Intact Rock Core Specimens under Varying
1.4 Thevaluesstatedininch-poundunitsaretoberegarded
States of Stress and Temperatures
as standard, except as noted below. The values given in
parentheses are mathematical conversions to SI units, which
3. Terminology
are provided for information only and are not considered
standard. Reporting of test results in units other than SI shall
3.1 Definitions:
not be regarded as nonconformance with this test method.
3.1.1 For terminology used in this test method, refer to
1.5 This standard does not purport to address all of the
Terminology D653.
safety problems, if any, associated with its use. It is the
3.2 Definitions of Terms Specific to This Standard:
responsibility of the user of this standard to establish appro-
3.2.1 deformation, n—displacement change in dimension of
priate safety and health practices and determine the applica-
the borehole due to changes in stress.
bility of regulatory limitations prior to use.
3.2.2 in situ stress, n—the stress levels and orientations
2. Referenced Documents
existing in the rock mass before excavation.
2.1 ASTM Standards:
3.2.3 principal plane, n—any plane in which the shear
D653Terminology Relating to Soil, Rock, and Contained
stresses are zero.
Fluids
3.2.4 principalstresses,n—thenormalstressesactingonthe
three principal planes and which are perpendicular to each
ThistestmethodisunderthejurisdictionofASTMCommitteeD18onSoiland
Rock and is the direct responsibility of Subcommittee D18.12 on Rock Mechanics. other.
Current edition approved Dec. 1, 2016. Published January 2017. Originally
3.2.4.1 Discussion—The major, intermediate or minor nor-
approved in 1986. Last previous edition approved in 2005 as D4623–05. DOI:
mal or principal stresses refers to the maximum, intermediate
10.1520/D4623-08.
Considerable information presented in this test method was taken from Bureau
and minor normal stresses occurring in the rock element.
of Mines Information Circular No. 8618, and Hooker, V.E., and Bickel, D.L.,
3.2.5 reverse case, n—inrockwhichtendstofractureeasily,
“Overcoring Equipment and Techniques Used in Rock Stress Determination,”
Denver Mining Research Center, Denver, CO, 1974.
“disc” or “poker chip” during overcoring, the borehole gauge
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
can be modified by replacing the standard housing with a
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
“reverse case” housing which allows the cantilever plungers to
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. be positioned very close to the start of the EX hole.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D4623 − 16
3.2.6 secondary principal stresses, n—the maximum, inter- now available and can go deeper; however, the pros and cons
mediate and minimum stresses for stress ellipsoids other than of each method need to be carefully compared to this test
the stress ellipsoid that contains the three maximum principal
method.
stresses.
5.3 It is also useful for obtaining stress characteristics of
existing concrete and rock structures, such as mass concrete
4. Summary of Test Method
dams, for safety (such as alkali aggregate issues), vetting of
4.1 Theovercoretestmeasuresthediametraldeformationof
computer models, and modification investigations.
a small-diameter borehole as it is removed from the surround-
ing stress field by coaxially coring a larger diameter hole. 5.4 This test method is difficult in rock with fracture
Deformation is measured across three diameters of the small
spacings of less than 5 in. (130 mm). A large number of tests
hole, spaced 60° apart, using a deformation gauge developed
may be required in order to obtain data.
by the U.S. Bureau of Mines. With knowledge of the rock
5.5 The rock tested is assumed to be homogeneous and
deformation moduli, the measured borehole deformation can
linearly elastic.The moduli of deformation and Poisson’s ratio
berelatedtothechangeinstressinaplaneperpendiculartothe
of the rock overcore are required for data reduction. The
borehole. This change in stress is assumed to be numerically
preferred method for determining modulus of deformation
equal, although opposite in sense to the stresses existing in the
values involves biaxially testing the recovered overcores, as
parent rock mass. Deformation measurements from three
described in Section 8. If this is not possible, values may be
nonparallel boreholes, together with rock deformation moduli,
determined from uniaxial testing of smaller cores in accor-
allow calculation of an estimate of the complete three-
dance with Test Method D7012. However, this generally
dimensional state of stress in the rock mass. Deformation
measurementsinonlyonedrillholedirectionwillonlygivethe decreasestheaccuracyofthestressdeterminationinallbutthe
secondary principal stresses unless something is known about most homogeneous and isotropic rock. Modulus of deforma-
one of the principal stress directions and the drill hole axis is
tion results may be used from other in situ tests, such as Test
aligned with that principal stress direction.
Methods D4394 and Test Method D4395, D4971 or other test
methods that can determine the modulus of deformation in
5. Significance and Use
specific directions.
5.1 Eithertheinsitustressesorthestressesasinfluencedby
5.6 The physical conditions present in three separate drill
an excavation may be determined. This test method is written
holes are assumed to prevail at one point in space to allow the
assuming testing will be done from an underground opening;
three-dimensional stress field to be estimated.This assumption
however, the same principles may be applied to testing in a
is difficult to verify, as rock material properties and the local
rock outcrop at the surface.
stress field can vary significantly over short distances. Confi-
5.2 Thistestmethodisgenerallyperformedatdepthswithin
denceinthisassumptionincreaseswithcarefulselectionofthe
50 ft (15 m) of the working face because of drilling difficulties
test site.
atgreaterdepths.Somedeepertestingwiththisgaugehasbeen
5.7 Local geologic features with mechanical properties
done, but should be considered developmental. This test
different from those of the surrounding rock can influence
method has a long and proven record and considered very
accurate relative to many other techniques, both new and old, significantly the local stress field. In general, these features, if
known to be present, should be avoided when selecting a test
out there. Other overcoring methods that use instruments that
are different, but follow much of the same basic concepts are site location. It is often important, however, to measure the
FIG. 1 Three-Component Borehole Deformation Gauge Showing Outer Housing Pulled Apart with Cable Assembly on Top Right; Three-
Component Pistom Assembly in Center and Outer Protective Cover on Left. Special Pliers for Working on Gauge at the Top, a Piston,
Dissassembled Piston and Washer (lower left), and a Transducer with Nut (lower right)
D4623 − 16
stress level on each side of a large fault. All boreholes at a borehole. Required accessories are special pliers, 0.005 and
single test station should be in the same formation or rock 0.015 in. (0.127 and 0.381 mm) thick, brass piston washers,
mass. and silicone grease.
6.1.2 Strain Readout Indicators—Three strain readout indi-
5.8 Since most overcoring is performed to measure in situ
cators are normally used to read the deformations.
stress levels, the boreholes should be drilled from a portion of
(Alternatively, one indicator with a switch and balance unit
the test opening and the testing performed at least three
may be used (see Fig. 2) or one indicator may be used in
excavation diameters from any free surface. The smallest
conjunction with a manual wire changing to obtain readings
opening that will accommodate the drilling equipment is
from the three axes.) These units need a full range digital
recommended; openings from 8 to 12 ft (2.4 to 3.6 m) in
readout limit of 40,000 indicator units. Indicators need to be
diameter have been found satisfactory.
−6
capable of measuring to an accuracy of 65×10 in.
−5 −6 −6
5.9 Aminimumofthreenonparallelboreholesisrequiredto
(13×10 mm) with a resolution of 1×10 in. (25×10
determine the complete stress tensor. The optimum angle each
mm). A calibration factor must be obtained for each axis to
hole makes with the other two (trihedral arrangement) is 90°.
relateindicatorunitstomicroinchesdeflection.Thecalibration
However, angles of 45° provide satisfactory results for deter-
factor for each axis will change proportionally with the gauge
mining all three principal stresses. Boreholes inclined upward
factor used. Normally, a gauge factor of 0.40 gives a good
are generally easier to work in than holes inclined downward,
balance between range and sensitivity. Fig. 2 shows a typical
particularly in fractured rock.
strain indicator, calibration jig, and a switching unit. Newer
data acquisition systems and microcomputer may be substi-
NOTE 2—The quality of the result produced by this standard is
dependent on the competence of the personnel performing it, and the tuted for the indicators.
suitability of the equipment and facilities used. Agencies that meet the
6.1.3 Cable—A shielded eight-wire conductor cable trans-
criteria of Practice D3740 are generally considered capable of competent
mits the strain measurements from the gauge to the strain
and objective testing/sampling/inspection/ and the like. Users of this
indicators. The length of cable required is the depth to the test
standard are cautioned that compliance with Practice D3740 does not in
position from the surface plus about 30 ft (10 m) to reach the
itself assure reliable results. Reliable results depend on many factors;
Practice D3740 provides a means of evaluating some of those factors.
strain indicators. A spare cable or an entire spare gauge and
cable should be considered if many tests are planned.
6. Apparatus
6.1.4 Calibration Jig—A jig (Fig. 2) used to calibrate the
6.1 Instrumentation:
BDG before and after testing with the use of two micrometer
6.1.1 Borehole Deformation Gauge—The borehole defor-
heads.
mation gauge is shown in Fig. 1 (in fractured rock, the
reverse-case modification of the gauge is recommended). The
More details of the gauge are described in: Hooker, V.E., Aggson, J.R., and
gauge is designed to measure diametral deformations during
Bickel, D.L., Improvements in the Three-Component Borehole Deformation Gauge
overcoring along three diameters, 60° apart in a plane perpen-
and Overcoring Techniques, Report of Investigation 7894, U.S. Bureau of Mines,
dicular to the walls of an EX (1 ⁄2-in. (38-mm) diameter) Washington, DC, 1984.
FIG. 2 The Calibration Jig (Left Side), with the BDG Inserted, Strain Readout Unit in Middle and a Switching and Balancing Unit (Right
Side)
D4623 − 16
6.1.5 Biaxial Modulus Chamber Apparatus—A steel cham- 7. Calibration and Standardization
ber incorporating an internal neoprene membrane which is
7.1 Gauge Calibration—Calibrate the BDG prior to begin-
sized to hold the rock overcore internally and which in
ning and end of the test program, or more frequently if
conjunction with a hand hydraulic pump and pressure gauge
conditions require. Also recalibrate the BDG if it has under-
and permits a known and sufficient biaxial pressure to be
goneseverevibration(especiallytothesignalcable),orifthere
applied hydraulically to the overcore sample while the BDG is
are any other reasons that exist to suspect that the gauge
inserted and deformation readings are taken. Ideally the maxi-
performance has changed. The recommended calibration pro-
mum pressure should be similar to the best estimates of the in
cedure is as follows:
situ rock stress, but usually should not exceed 3000 psi
7.1.1 Grease all gauge pistons with a light coat of silicone
(20MPa). The biaxial chamber is used to obtain data that is
grease and install them in the gauge.
then used for determining the deformation modulus of the
7.1.2 Place the gauge in the calibration jig as shown in Fig.
retrieved rock core. A schematic of the entire apparatus is
2,withthepistonsofthe Uaxisvisiblethroughthemicrometer
shown in Fig. 3.
holes of the jig. Tighten the wing nuts.
6.2 Orientation, Placement, and Retrieval Tools—
7.1.3 Install the two micrometer heads, and lightly tighten
6.2.1 Placement and retrieval tool or “J slot tool” as shown
the set screws.
in Fig. 4.
7.1.4 Set the strain indicators on “Full Bridge,” and then
6.2.2 PlacementorretrievalrodextensionsasshowninFig.
center the balance knob and set the gauge factor to correspond
4.
to the respective anticipated in-situ range and sensitivity
6.2.3 Orientation tool or “T” handle, also shown in Fig. 4.
requirements.Alowergaugefactorresultsinhighersensitivity.
6.2.4 Ascribingtool,formakinganorientationmarkonthe
The gauge factor used should be the same for calibration,
core for later biaxial testing, is optional. It consists of a
in-situ testing, and modulus tests.
bullet-shaped stainless steel head attached to a 3-ft (1-m) rod
7.1.5 Wire the gauge to the indicators as shown in Fig. 5 or
extension. Projecting perpendicularly from the stainless steel
to a switching and balance unit and one indicator.
head is a diamond stud. The stud is adjusted outward until a
7.1.6 Balance the indicator using the “Balance” knob (if
snug fit is achieved in the EX hole, so that a line is scratched
using three indicators).
along the borehole wall as the scribing tool is pushed inward.
7.1.7 Turn one micrometer in until the needle of the
6.2.5 Pajari alignment device for inserting into the hole to
indicator just starts to move.The micrometer is now in contact
determine the inclination. It consists of a floating compass and
with the piston. Repeat with the other micrometer.
an automatic locking device which locks the compass in
7.1.8 Rebalance the indicator.
position before retrieving it.
7.1.9 Record this no load indicator reading for the U axis.
6.3 Drilling Equipment—Adetailed description of the drill-
7.1.10 Turnineachmicrometer0.0160in.(0.406mm),ora
ing apparatus required is included in Annex A1.
total of 0.0320 in. (0.813 mm) displacement.
7.1.11 Balance the indicator and record the reading and the
6.4 Miscellaneous Equipment—Thisfieldoperationrequires
deflection.
a good set of assorted hand tools which should include a
solderingiron,solderandflux,heatgun,pliers,pipewrenches, 7.1.12 Wait 2 min to check the combined creep of the two
transducers.Creepshouldnotexceed20µin./in.(20µmm/mm)
adjustable wrenches, end wrenches, screwdrivers, Allen
wrenches, a hammer, electrical tape, a yardstick, carpenter’s in 2 min.
rule, chalk, indelible markers, stopwatch, and a thermometer. 7.1.13 Record the new reading.
FIG. 3 Schematic of Biaxial Modulus Chamber Test Apparatus for Overcore Rock Section
D4623 − 16
FIG. 4 Placement and Retrieval Tools. From top to bottom, “J” slot tool, Extension rod, and “T” handle.
FIG. 5 Wire Hookup to the Strain Readout Indicators
7.1.14 Back out each micrometer 0.0040 in. (0.102 mm) a 7.1.21 Loosen wing nuts, and align pistons of U axis with
total of 0.0080 in. (0.203 mm).
micrometer holes. Repeat the calibration procedure followed
7.1.15 Balance and record.
for the U and U axis.
1 2
7.1.16 Continue this procedure with the same increments
7.1.22 Determine the calibration factor for each axis as
until the initial point on the micrometer is reached. This zero
follows:
displacement will be the zero displacement reading for the
7.1.22.1 Subtract the zero displacement strain indicator
second run.
readings (last reading of each run) from the indicator reading
7.1.17 Repeat the operations described in 7.1.10 – 7.1.16.
for each deflection to establish the differences.
7.1.18 Loosen the wing nuts, and rotate the gauge to align
7.1.22.2 Subtract the difference in indicator units at 0.0080-
the piston of the U axis with the micrometer holes.
in.(0.203-mm)deflectionfromthedifferenceinindicatorunits
7.1.19 Retighten the wing nuts.
7.1.20 Repeat the operations described in 7.1.6 – 7.1.17. at 0.0320-in. (0.813-mm) deflection.
D4623 − 16
7.1.22.3 Divide the difference in deflection 0.0240 in. it, attach the regular 6-in. (152-mm) bit and barrel and extend
(0.610 mm) by the corresponding difference in indicator units the 6-in. (152-mm) hole to within 12 in. (305 mm) of the
just calculated to obtain the calibration factor for that axis.
desired test depth.
7.1.22.4 Repeat for the second cycle and take the mean as
8.2.4 Retrieve the core and insert the necessary length of
the calibration factor.
casing, including stabilizers.
7.1.22.5 SeeAppendixX1foranexampleofthecalibration
8.2.5 Insert the EX bit and reamer coupled to the EX core
for one axis, calibrated at a gauge factor of 0.40.
barrel and rods. Drill 2 to 7 ft (0.6 to 2.1 m) of EX hole.
8.2.6 Retrieve the EX core and inspect. Insert the scribing
8. Procedure
tool(ifthismethodoforientingthecoreisused)coupledtothe
8.1 The procedure for obtaining data to determine in-situ
rod extensions to the beginning of the EX hole. Attach the
stresses can be divided into two testing phases: (a) strain relief
orientationhandleandorientthescribemarkasdesired.Shove
measurementsin-situ,and(b)determinationofYoung’smodu-
the scribe straight down the hole. (If the scribe cannot be
lus of the rock by recompression in a biaxial chamber.
pushed down the hole, the diamond stud is projecting too far;
8.1.1 General—Holes of two sizes are drilled for the over-
adjust it inward. If the scribe feels loose, the stud must be
core test: an EX-size (1.5-in. (38-mm) diameter) hole for the
adjusted to project further.)When the scribe hits the bottom of
deformation gauge and a large-diameter overcore hole, gener-
theEXhole,slowlypullitbackupalongthesamescribemark.
ally 5.625-in. (143-mm) diameter core size and 6.00 in. (152
Ifjointsorfracturesintersecttheboreholewalls,theycanoften
mm) in diameter hole size. The two boreholes shall be
be detected. If fractures are detected, extend the hole and try
concentric to within 1.25 in. (32 mm) of the circumference of
again.WhentheEXholehasbeenscribed,removethescribing
the core diameter. All 6-in. (152-mm) drilling is done with
tool.
thin-walled diamond bits.Any pressure gauge or other meters
8.2.7 Tape together the ends of the cable from the BDG so
shall be functional and accurate to specifications.
no moisture can enter and thread the conductor cable through
8.2 Strain Relief Measurements:
the chuck and water swivel. Reconnect the wires to the strain
8.2.1 Test Planning:
indicator(s) exactly as during calibration.
8.2.1.1 Test Intervals—At least six tests per borehole are
8.2.8 Take zero deformation readings for each axis and
recommended beyond the zone of influence of the excavation.
record on the Field Data Sheet (Fig. 6) in the row labeled
In fractured rock, it may be necessary to test as often as
“zero”andinthethreecolumnslabeled U , U ,and U .Ifonly
1 2 3
possible to obtain a sufficient amount of usable data. In any
onestrainindicatorisbeingused,aswitchingunitisnecessary.
case, begin the testing beyond the zone of damage caused by
If a switching unit is not available, the wires must be changed
the excavation of the test adit, as determined from prior
for each axis. Check each axis by applying slight finger
exploratory drilling or the initial coring of the overcore hole.
pressuretoopposingpistonsandreleasing.Thebalanceneedle
8.2.1.2 Coaxial Requirements—The EX and large diameter
should deflect, then return to the balanced position. Check
boreholes shall be concentric to within 1.25 in. (32 mm) of the
tightness of cable connection.
circumference of the core diameter. When this tolerance is
8.2.9 Engage the orientation pins of the BDG with the
exceeded, overcore out the rock containing the existing EX
placementtoolusingaclockwisemotion.Securetheconductor
hole and restart drilling.
cable with the wire retainer clip in the placement tool. Make
8.2.1.3 Test Location—If possible, locate the plane of de-
sure the orientation pins of the BDG are aligned with the U
formation measurements at least one diameter of the large
axis.Pushthegaugethroughthestabilizertubeandabout9in.
borehole ahead of the larger hole at the start of overcoring. If
(229mm)intotheEXhole.Withthegaugeattestdepth,orient
this is not feasible, for instance because of fractures, locate the
the U axis along the scribe mark by turning clockwise. If the
plane of measurements as far ahead of the large borehole as
BDG feels too loose or too tight in the EX hole, it must be
possible. Do not locate the borehole deformation gauge so that
removed. If too tight, remove one washer from one piston of
the measuring buttons and support springs are located in
each axis and try again. If too loose, add one washer to one
different blocks of rock, which will undergo differential move-
piston of each. To add or remove a washer, pull the piston out
ment when overcored. The exact test location may be deter-
with the special pliers, unscrew the two piston halves, remove
mined from examination of the EX core. In highly fractured
or add a washer and screw back together. Be careful not to
rock, examination of the EX borehole with a borescope or
damagetheOringandreinstallthepistoninthegauge.Dothis
borehole camera is recommended before testing.
initially to only one piston in each diametral pair. If the gauge
8.2.2 Drill Setup—To obtain high-quality data from the
is still too tight or loose, repeat for the remaining pistons.
overcore test, it is important to minimize drilling vibrations
8.2.10 Theorientationoftheboreholedeformationgaugein
during the test.To accomplish this, support the drill to prevent
any vibratory motion or misalignment while drilling. Rock a particular position is not required; a variety of orientations
bolts, roof jacks, timber posts and wedges, and other support arerecommendedtominimizesystematicerrorsanduncertain-
systems have been used successfully. Start approximately tiesduetorockanisotropy.Eachorientation,however,shallbe
horizontal holes 5° upward from horizontal to facilitate re- accuratelymeasuredtowithin 65°.Thismaybeaccomplished
moval of water and cuttings. by a measurement device on the end of the setting tools, by
examining the gauge in the borehole with a low-power
8.2.3 To start a test borehole, use a 6-in. (152-mm) starter
barrel. Once the barrel has been advanced sufficiently, remove telescope, or by other suitable means.
D4623 − 16
FIG. 6 Field Data Sheet
8.2.11 With the gauge installed at the test depth and cor- some secured object. Only slight tension in the rope is
rectly oriented, check the bias of the gauge on the strain necessary in order to keep the cable from twisting during
indicators. The bias set on each component should be between drilling.
13,000 and 20,000 indicator units with a gauge factor of 0.40 8.2.14 Turn on the water. Allow approximately 10 min for
forovercoringstrainrelieftests.Forrecompressiontestsinthe gauge, water, and rock to reach temperature equilibrium. The
biaxial chamber, the bias should be between 8,000 and 12,000 circuits in the borehole deformation gauge have been designed
indicatorunitswithagaugefactorof0.40.Withagaugefactor to minimize thermal drift. Measure the temperature of the
of 1.50, the bias should be between 4,500 and 7,000 indicator drilling water at the beginning and end of each test and at any
units for overcoring tests and between 1,800 and 3,600 other time when thermal drift is suspected. To better monitor
indicator units for recompression tests. Take care to avoid thermal effects, install a temperature sensing device like a
overloadingthetransducers.Maximumloadonanycomponent thermistorinthegaugeitself.Whenthedriftcriterionhasbeen
shouldnotexceed20,000indicatorunitswithagaugefactorof satisfied, obtain new zero readings for each axis.
0.40 and 4,560 units with a gauge factor of 1.50. 8.2.15 With the 6-in. (152-mm) bit resting on the bottom of
8.2.12 Turn the placement tool counterclockwise approxi- the hole, tape a yardstick to the drill so as to monitor drilling
mately 60° to disengage it from the BDG and remove the tool. advancement as overcoring proceeds; check the advance rate
(When retrieving the BDG, this procedure is reversed.) by timing with a stopwatch. Alternatively, the exposed casing
8.2.13 Pulltheslackconductorcablethroughthechuckand on the drill hydraulic guides may be marked at ⁄2-in. (13-mm)
water swivel. Avoid excess tension in the cable or the gauge increments to regulate the advance rate.
maybepulledoutoftheEXhole.Closethedrillandcouplethe 8.2.16 Start overcoring at an approximate penetration rate
casing to the chuck adaptor. Tie off the cable with a rope to of ⁄2 in. (13 mm)/40 s and a chuck speed of 50 rpm. Use a
D4623 − 16
stopwatchtocalibratethedrilltothisrate.Each ⁄2-in.(13-mm) overcoringandsmoothcurvesfromthedataindicatethatatest
penetration should be signaled to the recorder who records the is successful.
indicator readings for each axis on the field data sheet. 8.2.24 Repeat this procedure for each additional test.
Continuously overcore approximately 12 to 18 in. (305 to 457
8.3 Procedure for Determining Young’s Modulus of Elastic-
mm) at this rate. If the core breaks during overcoring, the 5
ity of the Rock Core:
needles on the strain indicators will fluctuate erratically or the
8.3.1
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D4623 − 08 D4623 − 16
Standard Test Method for
Determination of In Situ Stress in Rock Mass by Overcoring
Method—USBM Method—Three Component Borehole
Deformation Gauge
This standard is issued under the fixed designation D4623; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 This test method covers the determination of the ambient local stresses (principal and secondary) in a rock mass and the
equipment required to perform in situ stress tests using a three-component borehole deformation gauge (BDG) that was developed
by the U.S. Bureau of Mines (USBM); see Note 1.
1.2 The test procedure and method of data reduction are described, including the theoretical basis and assumptions involved in
the calculations.
1.3 This test method covers the determination of the ambient local stresses in a rock mass and the equipment required to perform
in situ stress tests using a three-component borehole deformation gauge (BDG). The test procedure and method of data reduction
are described, including the theoretical basis and assumptions involved in the calculations. A section is included on troubleshooting
equipment malfunctions.
NOTE 1—The gauge used in this test method is commonly referred to as a USBM gauge (U.S. Bureau of Mines three-component borehole deformation
gauge).
NOTE 1—The gauge used in this test method is commonly referred to by users as a USBM gauge (U.S. Bureau of Mines three-component borehole
deformation gauge).
1.4 The values stated in inch-pound units are to be regarded as standard. No other units of measurement are included in this
standard.standard, except as noted below. The values given in parentheses are mathematical conversions to SI units, which are
provided for information only and are not considered standard. Reporting of test results in units other than SI shall not be regarded
as nonconformance with this test method.
1.5 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
D653 Terminology Relating to Soil, Rock, and Contained Fluids
D3740 Practice for Minimum Requirements for Agencies Engaged in Testing and/or Inspection of Soil and Rock as Used in
Engineering Design and Construction
D4394 Test Method for Determining In Situ Modulus of Deformation of Rock Mass Using Rigid Plate Loading Method
D4395 Test Method for Determining In Situ Modulus of Deformation of Rock Mass Using Flexible Plate Loading Method
D4971 Test Method for Determining In Situ Modulus of Deformation of Rock Using Diametrically Loaded 76-mm (3-in.)
Borehole Jack
D6026 Practice for Using Significant Digits in Geotechnical Data
D7012 Test Methods for Compressive Strength and Elastic Moduli of Intact Rock Core Specimens under Varying States of
Stress and Temperatures
This test method is under the jurisdiction of ASTM Committee D18 on Soil and Rock and is the direct responsibility of Subcommittee D18.12 on Rock Mechanics.
Current edition approved July 1, 2008Dec. 1, 2016. Published July 2008January 2017. Originally approved in 1986. Last previous edition approved in 2005 as D4623 – 05.
DOI: 10.1520/D4623-08.
Considerable information presented in this test method was taken from Bureau of Mines Information Circular No. 8618, and Hooker, V.E., and Bickel, D.L., “Overcoring
Equipment and Techniques Used in Rock Stress Determination,” Denver Mining Research Center, Denver, CO, 1974.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D4623 − 16
3. Terminology
3.1 Definitions—Definitions: See Terminology D653 for general definitions.
3.1.1 For terminology used in this test method, refer to Terminology D653.
3.2 Definitions:Definitions of Terms Specific to This Standard:
3.2.1 deformation—deformation, n—displacement change in dimension of the borehole due to changes in stress.
3.2.2 in situ stress—stress, n—the stress levels and orientations existing in the rock mass before excavation.
3.2.3 principal plane, n—any plane in which the shear stresses are zero.
3.2.4 principal stresses, n—the normal stresses acting on the three principal planes and which are perpendicular to each other.
3.2.4.1 Discussion—
The major, intermediate or minor normal or principal stresses refers to the maximum, intermediate and minor normal stresses
occurring in the rock element.
3.2.5 reverse case, n—in rock which tends to fracture easily, “disc” or “poker chip” during overcoring, the borehole gauge can
be modified by replacing the standard housing with a “reverse case” housing which allows the cantilever plungers to be positioned
very close to the start of the EX hole.
3.2.6 secondary principal stresses, n—the maximum, intermediate and minimum stresses for stress ellipsoids other than the
stress ellipsoid that contains the three maximum principal stresses.
4. Summary of Test Method
4.1 The overcore test measures the diametral deformation of a small-diameter borehole as it is removed from the surrounding
stress field by coaxially coring a larger diameter hole. Deformation is measured across three diameters of the small hole, spaced
60° apart, using a deformation gauge developed by the U.S. Bureau of Mines. With knowledge of the rock deformation moduli,
the measured borehole deformation can be related to the change in stress in a plane perpendicular to the borehole. This change
in stress is assumed to be numerically equal, although opposite in sense to the stresses existing in the parent rock mass.
Deformation measurements from three nonparallel boreholes, together with rock deformation moduli, allow calculation of an
estimate of the complete three-dimensional state of stress in the rock mass. Deformation measurements in only one drill hole
direction will only give the secondary principal stresses unless something is known about one of the principal stress directions and
the drill hole axis is aligned with that principal stress direction.
5. Significance and Use
5.1 Either virgin the in situ stresses or the stresses as influenced by an excavation may be determined. This test method is written
assuming testing will be done from an underground opening; however, the same principles may be applied to testing in a rock
outcrop at the surface.
5.2 This test method is generally performed at depths within 50 ft (15 m) of the working face because of drilling difficulties
at greater depths. Some deeper testing with this gauge has been done, but should be considered developmental. It is also useful
for obtaining stress characteristics of existing concrete and rock structures for safety and modification investigations. This test
method has a long and proven record and considered very accurate relative to many other techniques, both new and old, out there.
Other overcoring methods that use instruments that are different, but follow much of the same basic concepts are now available
and can go deeper; however, the pros and cons of each method need to be carefully compared to this test method.
5.3 It is also useful for obtaining stress characteristics of existing concrete and rock structures, such as mass concrete dams, for
safety (such as alkali aggregate issues), vetting of computer models, and modification investigations.
5.4 This test method is difficult in rock with fracture spacings of less than 5 in. (130 mm). A large number of tests may be
required in order to obtain data.
5.5 The rock tested is assumed to be homogeneous and linearly elastic. The moduli of deformation and Poisson’s ratio of the
rock overcore are required for data reduction. The preferred method for determining modulus of deformation values involves
biaxially testing the recovered overcores, as described in Section 8. If this is not possible, values may be determined from uniaxial
testing of smaller cores in accordance with Test Method D7012. However, this generally decreases the accuracy of the stress
determination in all but the most homogeneous rock. Results and isotropic rock. Modulus of deformation results may be used from
other in situ tests, such as Test MethodMethods D4394 and Test Method D4395., D4971 or other test methods that can determine
the modulus of deformation in specific directions.
5.6 The physical conditions present in three separate drill holes are assumed to prevail at one point in space to allow the
three-dimensional stress field to be estimated. This assumption is difficult to verify, as rock material properties and the local stress
field can vary significantly over short distances. Confidence in this assumption increases with careful selection of the test site.
D4623 − 16
FIG. 1 Special Pliers, the Bureau of Mines’ Three-Component Borehole Gauge, a Piston, Disassembled Piston and Washer, Three-
Component Borehole Deformation Gauge Showing Outer Housing Pulled Apart with Cable Assembly on Top Right; Three-Component
Pistom Assembly in Center and Outer Protective Cover on Left. Special Pliers for Working on Gauge at the Top, a Piston, Dissas-
sembled Piston and Washer (lower left), and a Transducer with Nut (lower right)
5.7 Local geologic features with mechanical properties different from those of the surrounding rock can influence significantly
the local stress field. In general, these features, if known to be present, should be avoided when selecting a test site location. It
is often important, however, to measure the stress level on each side of a large fault. All boreholes at a single test station should
be in the same formation.formation or rock mass.
5.8 Since most overcoring is performed to measure undisturbed in situ stress levels, the boreholes should be drilled from a
portion of the test opening and the testing performed at least three excavation diameters from any free surface. The smallest
opening that will accommodate the drilling equipment is recommended; openings from 8 to 12 ft (2.4 to 3.6 m) in diameter have
been found satisfactory.
5.9 A minimum of three nonparallel boreholes is required to determine the complete stress tensor. The optimum angle each hole
makes with the other two (trihedral arrangement) is 90°. However, angles of 45° provide satisfactory results for determining all
three principal stresses. Boreholes inclined upward are generally easier to work in than holes inclined downward, particularly in
fractured rock.
NOTE 2—The quality of the result produced by this standard is dependent on the competence of the personnel performing it, and the suitability of the
equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective
testing/sampling/inspection/ and the like. Users of this standard are cautioned that compliance with Practice D3740 does not in itself assure reliable
results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.
6. Apparatus
6.1 Instrumentation:
6.1.1 Borehole Deformation Gauge—The USBM borehole deformation gauge is shown in Fig. 1 (in fractured rock, the
reverse-case modification of the gauge is recommended). The gauge is designed to measure diametral deformations during
overcoring along three diameters, 60° apart in a plane perpendicular to the walls of an EX (1 ⁄2-in. (38-mm) diameter) borehole.
Required accessories are special pliers, 0.005 and 0.015 in. (0.127 and 0.381 mm) thick, brass piston washers, and silicone grease.
6.1.2 Strain Readout Indicators—Three strain readout indicators normally are normally used to read the deformations.
(Alternatively, one indicator with a switch and balance unit may be used (see Fig. 2) or one indicator may be used in conjunction
with a manual wire changing to obtain readings from the three axes.) These units need a full range digital readout limit of
−6 −5
40 00040,000 indicator units. Indicators need to be capable of measuring to an accuracy of 65 × 10 in. (13 × 10 mm) with a
−6 −6
resolution of 1 × 10 in. (25 × 10 mm). A calibration factor must be obtained for each axis to relate indicator units to
microinches deflection. The calibration factor for each axis will change proportionally with the gauge factor used. Normally, a
gauge factor of 0.40 gives a good balance between range and sensitivity. Fig. 2 shows a typical strain indicator, calibration jig, and
a switching unit. Newer data acquisition systems and microcomputer may be substituted for the indicators.
6.1.3 Cable—A shielded eight-wire conductor cable transmits the strain measurements from the gauge to the strain indicators.
The length of cable required is the depth to the test position from the surface plus about 30 ft (10 m) to reach the strain indicators.
A spare cable or an entire spare gauge and cable should be considered if many tests are planned.
6.1.4 Orientation and Placement Tools— The orientation and placement tools consist of:
6.1.4.1 Placement tool or “J slot tool” as shown in Fig. 3.
More details of the gauge are described in: Hooker, V.E., Aggson, J.R., and Bickel, D.L., Improvements in the Three-Component Borehole Deformation Gauge and
Overcoring Techniques, Report of Investigation 7894, U.S. Bureau of Mines, Washington, DC, 1984.
D4623 − 16
FIG. 2 The Calibration Device (Left Side) Jig (Left Side), with the BDG Inserted, Strain Readout Unit in Middle and a Switching and Bal-
ancing Unit (Right Side)
6.1.4.2 Placement rod extensions as shown in Fig. 3.
6.1.4.3 Orientation tool or “T handle,” also shown in Fig. 3.
6.1.4.4 A scribing tool, for making an orientation mark on the core for later biaxial testing, is optional. It consists of a
bullet-shaped stainless steel head attached to a 3-ft (1-m) rod extension. Projecting perpendicularly from the stainless steel head
is a diamond stud. The stud is adjusted outward until a snug fit is achieved in the EX hole, so that a line is scratched along the
borehole wall as the scribing tool is pushed inward.
6.1.4.5 Pajari alignment device for inserting into the hole to determine the inclination. It consists of a floating compass and an
automatic locking device which locks the compass in position before retrieving it.
6.1.4 Calibration Jig—A calibration jig (Fig. 2) is used to calibrate the BDG before and after testing. testing with the use of
two micrometer heads.
6.1.5 Biaxial Chamber—Modulus Chamber Apparatus—A biaxial chamber with steel chamber incorporating an internal
neoprene membrane which is sized to hold the rock overcore internally and which in conjunction with a hand hydraulic pump and
pressure gauge is used to determine and permits a known and sufficient biaxial pressure to be applied hydraulically to the overcore
sample while the BDG is inserted and deformation readings are taken. Ideally the maximum pressure should be similar to the best
estimates of the in situ rock stress, but usually should not exceed 3000 psi (20MPa). The biaxial chamber is used to obtain data
that is then used for determining the deformation modulus of the retrieved rock core. A schematic of the entire apparatus is shown
in Fig. 43.
6.2 Orientation, Placement, and Retrieval Tools—
6.2.1 Placement and retrieval tool or “J slot tool” as shown in Fig. 4.
6.2.2 Placement or retrieval rod extensions as shown in Fig. 4.
6.2.3 Orientation tool or “T” handle, also shown in Fig. 4.
6.2.4 A scribing tool, for making an orientation mark on the core for later biaxial testing, is optional. It consists of a
bullet-shaped stainless steel head attached to a 3-ft (1-m) rod extension. Projecting perpendicularly from the stainless steel head
is a diamond stud. The stud is adjusted outward until a snug fit is achieved in the EX hole, so that a line is scratched along the
borehole wall as the scribing tool is pushed inward.
6.2.5 Pajari alignment device for inserting into the hole to determine the inclination. It consists of a floating compass and an
automatic locking device which locks the compass in position before retrieving it.
6.3 Drilling Equipment—A detailed description of the drilling apparatus required is included in Annex A1.
6.4 Miscellaneous Equipment—This field operation requires a good set of assorted hand tools which should include a soldering
iron, solder and flux, heat gun, pliers, pipe wrenches, adjustable wrenches, end wrenches, screwdrivers, allenAllen wrenches, a
hammer, electrical tape, a yardstick, carpenter’s rule, chalk, indelible markers, stopwatch, and a thermometer.
D4623 − 16
FIG. 43 Schematic: Biaxial Test ApparatusSchematic of Biaxial Modulus Chamber Test Apparatus for Overcore Rock Section
FIG. 34 Placement and Retrieval ToolTools. From top to bottom, “J” slot tool, Extension rod, and “T” handle.
7. Calibration and Standardization
7.1 Gauge Calibation—Calibration—Calibrate the BDG prior to beginning and end of the test program, or more frequently if
conditions require. Also recalibrate the BDG if it has undergone severe vibration (especially to the signal cable), or if there are
any other reasons that exist to suspect that the gauge performance has changed. The recommended calibration procedure is as
follows:
7.1.1 Grease all gauge pistons with a light coat of silicone grease and install them in the gauge.
7.1.2 Place the gauge in the calibration jig as shown in Fig. 2, with the pistons of the U axis visible through the micrometer
holes of the jig. Tighten the wing nuts.
7.1.3 Install the two micrometer heads, and lightly tighten the set screws.
7.1.4 Set the strain indicators on “Full Bridge,” and then center the balance knob and set the gauge factor to correspond to the
respective anticipated in-situ range and sensitivity requirements. A lower gauge factor results in higher sensitivity. The gauge factor
used should be the same for calibration, in-situ testing, and modulus tests.
7.1.5 Wire the gauge to the indicators as shown in Fig. 5 or to a switching and balance unit and one indicator.
7.1.6 Balance the indicator using the “Balance” knob (if using three indicators).
D4623 − 16
FIG. 5 Wire Hookup to Model P-350 the Strain Readout Indicators
7.1.7 Turn one micrometer in until the needle of the indicator just starts to move. The micrometer is now in contact with the
piston. Repeat with the other micrometer.
7.1.8 Rebalance the indicator.
7.1.9 Record this no load indicator reading for the U axis.
7.1.10 Turn in each micrometer 0.0160 in. (0.406 mm), or a total of 0.0320 in. (0.813 mm) displacement.
7.1.11 Balance the indicator and record the reading and the deflection.
7.1.12 Wait 2 min to check the combined creep of the two transducers. Creep should not exceed 20 μin./in. (20 μmm/mm) in
2 min.
7.1.13 Record the new reading.
7.1.14 Back out each micrometer 0.0040 in. (0.102 mm) a total of 0.0080 in. (0.203 mm).
7.1.15 Balance and record.
7.1.16 Continue this procedure with the same increments until the initial point on the micrometer is reached. This zero
displacement will be the zero displacement reading for the second run.
7.1.17 Repeat the operations described in 7.1.10 – 7.1.16.
7.1.18 Loosen the wing nuts, and rotate the gauge to align the piston of the U axis with the micrometer holes.
7.1.19 Retighten the wing nuts.
7.1.20 Repeat the operations described in 7.1.6 – 7.1.17.
7.1.21 Loosen wing nuts, and align pistons of U axis with micrometer holes. Repeat the calibration procedure followed for the
U and U axis.
1 2
7.1.22 Determine the calibration factor for each axis as follows:
7.1.22.1 Subtract the zero displacement strain indicator readings (last reading of each run) from the indicator reading for each
deflection to establish the differences.
7.1.22.2 Subtract the difference in indicator units at 0.0080-in. (0.203-mm) deflection from the difference in indicator units at
0.0320-in. (0.813-mm) deflection.
7.1.22.3 Divide the difference in deflection 0.0240 in. (0.610 mm) by the corresponding difference in indicator units just
calculated to obtain the calibration factor for that axis.
7.1.22.4 Repeat for the second cycle and take the mean as the calibration factor.
7.1.22.5 See Appendix X1 for an example of the calibration for one axis, calibrated at a gauge factor of 0.40.
8. Procedure
8.1 The procedure for obtaining data to determine in-situ stresses can be divided into two testing phases: (a) strain relief
measurements in-situ, and (b) determination of Young’s modulus of the rock by recompression in a biaxial chamber.
8.1.1 General—Holes of two sizes are drilled for the overcore test: an EX-size (1.5-in. (38-mm) diameter) hole for the
deformation gauge and a large-diameter overcore hole, generally 5.625-in. (143-mm) diameter core size and 6.00 in. (152 mm)
in diameter hole size. The two boreholes shall be concentric to within 1.25 in. (32 mm) of the circumference of the core diameter.
All 6-in. (152-mm) drilling is done with thin-walled diamond bits. Any pressure gauge or other meters shall be functional and
accurate to specifications.
8.2 Strain Relief Measurements : Measurements:
D4623 − 16
8.2.1 Test Planning:
8.2.1.1 Test Intervals—At least six tests per borehole are recommended beyond the zone of influence of the excavation. In
fractured rock, it may be necessary to test as often as possible to obtain a sufficient amount of usable data. In any case, begin the
testing beyond the zone of damage caused by the excavation of the test adit, as determined from prior exploratory drilling or the
initial coring of the overcore hole.
8.2.1.2 Coaxial Requirements—The EX and large diameter boreholes shall be concentric to within 1.25 in. (32 mm) of the
circumference of the core diameter. When this tolerance is exceeded, overcore out the rock containing the existing EX hole and
restart drilling.
8.2.1.3 Test Location—If possible, locate the plane of deformation measurements at least one diameter of the large borehole
ahead of the larger hole at the start of overcoring. If this is not feasible, for instance because of fractures, locate the plane of
measurements as far ahead of the large borehole as possible. Do not locate the borehole deformation gauge so that the measuring
buttons and support springs are located in different blocks of rock, which will undergo differential movement when overcored. The
exact test location may be determined from examination of the EX core. In highly fractured rock, examination of the EX borehole
with a borescope or borehole camera is recommended before testing.
8.2.2 Drill Setup—To obtain high-quality data from the overcore test, it is important to minimize drilling vibrations during the
test. To accomplish this, support the drill to prevent any vibratory motion or misalignment while drilling. Rock bolts, roof jacks,
timber posts and wedges, and other support systems have been used successfully. Start approximately horizontal holes 5° upward
from horizontal to facilitate removal of water and cuttings.
8.2.3 To start a test borehole, use a 6-in. (152-mm) starter barrel. Once the barrel has been advanced sufficiently, remove it,
attach the regular 6-in. (152-mm) bit and barrel and extend the 6-in. (152-mm) hole to within 12 in. (305 mm) of the desired test
depth.
8.2.4 Retrieve the core and insert the necessary length of casing, including stabilizers.
8.2.5 Insert the EX bit and reamer coupled to the EX core barrel and rods. Drill 2 to 7 ft (0.6 to 2.1 m) of EX hole.
8.2.6 Retrieve the EX core and inspect. Insert the scribing tool (if this method of orienting the core is used) coupled to the rod
extensions to the beginning of the EX hole. Attach the orientation handle and orient the scribe mark as desired. Shove the scribe
straight down the hole. (If the scribe cannot be pushed down the hole, the diamond stud is projecting too far; adjust it inward. If
the scribe feels loose, the stud must be adjusted to project further.) When the scribe hits the bottom
of the EX hole, slowly pull it back up along the same scribe mark. If joints or fractures intersect the borehole walls, they can often
be detected. If fractures are detected, extend the hole and try again. When the EX hole has been scribed, remove the scribing tool.
8.2.7 Tape together the ends of the cable from the BDG so no moisture can enter and thread the conductor cable through the
chuck and water swivel. Reconnect the wires to the strain indicator(s) exactly as during calibration.
8.2.8 Take zero deformation readings for each axis and record on the Field Data Sheet (Fig. 6) in the row labeled “zero” and
in the three columns labeled U , U , and U . If only one strain indicator is being used, a switching unit is necessary. If a switching
1 2 3
unit is not available, the wires must be changed for each axis. Check each axis by applying slight finger pressure to opposing
pistons and releasing. The balance needle should deflect, then return to the balanced position. Check tightness of cable connection.
8.2.9 Engage the orientation pins of the BDG with the placement tool using a clockwise motion. Secure the conductor cable
with the wire retainer clip in the placement tool. Make sure the orientation pins of the BDG are aligned with the U axis. Push
the gauge through the stabilizer tube and about 9 in. (229 mm) into the EX hole. With the gauge at test depth, orient the U axis
along the scribe mark by turning clockwise. If the BDG feels too loose or too tight in the EX hole, it must be removed. If too tight,
remove one washer from one piston of each axis and try again. If too loose, add one washer to one piston of each. To add or remove
a washer, pull the piston out with the special pliers, unscrew the two piston halves, remove or add a washer and screw back
together. Be careful not to damage the O ring and reinstall the piston in the gauge. Do this initially to only one piston in each
diametral pair. If the gauge is still too tight or loose, repeat for the remaining pistons.
8.2.10 The orientation of the borehole deformation gauge in a particular position is not required; a variety of orientations are
recommended to minimzeminimize systematic errors and uncertainties due to rock anisotropy. Each orientation, however, shall be
accurately measured to within 65°. This may be accomplished by a measurement device on the end of the setting tools, by
examining the gauge in the borehole with a low-power telescope, or by other suitable means.
8.2.11 With the gauge installed at the test depth and correctly oriented, check the bias of the gauge on the strain indicators. The
bias set on each component should be between 13 00013,000 and 20 00020,000 indicator units with a gauge factor of 0.40 for
overcoring strain relief tests. For recompression tests in the biaxial chamber, the bias should be between 8 0008,000 and
12 00012,000 indicator units with a gauge factor of 0.40. With a gauge factor of 1.50, the bias should be between 45004,500 and
70007,000 indicator units for overcoring tests and between 18001,800 and 36003,600 indicator units for recompression tests. Take
care to avoid overloading the transducers. Maximum load on any component should not exceed 20 00020,000 indicator units with
a gauge factor of 0.40 and 45604,560 units with a gauge factor of 1.50.
8.2.12 Turn the placement tool counterclockwise approximately 60° to disengage it from the BDG and remove the tool. (When
retrieving the BDG, this procedure is reversed.)
D4623 − 16
FIG. 6 Field Data Sheet
8.2.13 Pull the slack conductor cable through the chuck and water swivel. Avoid excess tension in the cable or the gauge may
be pulled out of the EX hole. Close the drill and couple the casing to the chuck adaptor. Tie off the cable with a rope to some
secured object. Only slight tension in the rope is necessary in order to keep the cable from twisting during drilling.
8.2.14 Turn on the water. Allow approximately 10 min for gauge, water, and rock to reach temperature equilibrium. The circuits
in the borehole deformation gauge have been designed to minimize thermal drift. Measure the temperature of the drilling water
at the beginning and end of each test and at any other time when thermal drift is suspected. To better monitor thermal effects, install
a temperature sensing device like a thermistor in the gauge itself. When the drift criterion has been satisfied, obtain new zero
readings for each axis.
8.2.15 With the 6-in. (152-mm) bit resting on the bottom of the hole, tape a yardstick to the drill so as to monitor drilling
advancement as overcoring proceeds; check the advance r
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